There is a general need in the X-ray field to provide better radiation measurements. It has been previously found that measuring the radiation in terms of peak kVp provides one practical measurement of X-ray beam quality.
Existing methods for measuring the voltage applied to an X-ray tube may be classified as either direct or indirect methods. Instruments for providing direct method measurements require electrical connections to the X-ray tube, to the high voltage transformer, or to other components in the high voltage circuit. The direct method necessitates alteration of the high voltage circuits; and, the instruments utilized with direct measurement methods are relatively expensive. Indirect methods, instead of making connection in the high voltage circuit, rely on measurement of signals that are related in a known fashion to the voltage applied to the X-ray tube. A number of indirect methods are known and used and most involve measurements of the X-ray beam emanating from the tube. Known indirect methods of voltage measurement require some user interpretation of data; some require special training and some methods require sophisticated and/or expensive equipment.
The invention discloses a unique indirect method of measuring the voltage applied to an X-ray tube which overcomes various disadvantages associated with existing indirect methods for measuring the voltage applied to an X-ray tube. The present invention provides instantaneous readout of voltage, utilizing compact, relatively inexpensive equipment which can be readily operated without requiring specialized training of the operator. Further the present invention determines the voltage from one exposure from the X-ray tube. Another technique for measuring X-ray beam quality which has found general acceptance is the measurement of the so called half-value thickness of half-values layer (HVL) of an X-ray beam. An existing technique for measuring HVL relies on having a single dosimeter, multiple filters and multiple X-ray exposures. Generally, a single exposure without added filtration is first made to obtain a reference exposure value. Additional exposures are then made, each exposure being made with a different thickness of filtration added between the X-ray tube and dosimeter until more than 50% of the beam is attenuated by the filter material. X-ray radiation is then plotted as a function of added filtration. The filtration required to reduce the beam by 50% of filtration is defined to be the half-value layer.
The present invention generally utilizes the principal of measuring beam quality by absorption in two filters. An article "Inferential kilovoltmeter" (subtitled "Measuring X-ray Kilovoltage by Absorption in Two Filters") by Robert R. Newell, M.D., and George C. Henry, M.D., presented as an Exhibit at the Thirty-Ninth Annual Meeting of the Radiological Society of North America, Chicago, Ill., Dec. 13-18, 1953 describes basic principals of the method. The article entitled "A Simple Method For Measuring Peak Voltage in Diagnostic Roentgen Equipment" by Russell H. Morgan, M.D., in the AMERICAN JOURNAL OF ROENTGENOLOGY AND RADIUM THERAPY, Vol. 1.11., No. 2, Aug. 1944 also describes a method for measuring kVp using two filters.
However, while general methods for measuring beam quality by utilizing absorption in two filters have been known, the present invention specifically provides a practical system for providing an accurate measurement of the X-ray beam by absorption techniques. The present invention also refines and quantitates the technique by deriving and elucidating relationships between kVp and detector signals.